Rechargeable DC power supplies are useful in many types of applications. For example, cellular towers and other stationary applications use rechargeable DC power supplies as an uninterruptible power source. The rechargeable DC power supplies provide backup power during a main grid outage.
Referring now to FIG. 1, an example application of rechargeable DC power supplies is shown. A direct current (DC) power supply 2 receives power from a main grid 4. The DC power supply 2 generally provides power to a load 6 and to a plurality of rechargeable DC power supplies 8-1, 8-2, . . . , 8-N, referred to collectively as rechargeable DC power supplies 8. Each rechargeable DC power supply 8 includes a corresponding battery 10 and charging control circuit 12.
The battery 10 provides a voltage that is less than a load voltage needed by the load 6. If the DC power supply 2 becomes inoperative, the series-connected rechargeable DC power supplies 8 provide the load voltage. However, because the rechargeable DC power supplies are connected in series, the load 6 may not receive sufficient voltage if one of more of the rechargeable DC power supplies 8 is in a discharged or open circuit condition.